The present invention relates to electronics and, more particularly, to output impedance-matching networks for RF power amplifiers and the like.
An RF power amplifier, for example, for a base station, is typically implemented with input and output impedance-matching networks respectively connected to the input and output nodes of the amplifier's active circuitry to match the input and output impedances of the RF power amplifier with the impedances of components respectively connected upstream and downstream of the RF power amplifier.
FIG. 1 is a schematic circuit diagram of a conventional, integrated RF power amplifier 100 having an input node 110 that receives an RF input signal 115 and presents a corresponding amplified RF output signal 145 at an amplifier output node 150. In particular, the RF input signal 115 at the amplifier input node 110 is applied to an input impedance-matching network 120, and a resulting filtered RF signal 125 is applied to the input (e.g., gate) of an active device 130. A resulting amplified RF signal 135 at the output (e.g., drain) of the active device 130 is applied to an output impedance-matching network 140, and the resulting filtered RF signal, i.e., the amplified RF output signal 145, is presented at the amplifier output node 150. Although the active device 130 is represented in FIG. 1 as a single power transistor, the active device 130 is sometimes also implemented using more involved active circuitry comprising a suitable configuration of multiple transistors and other electronic components.
As shown in FIG. 1, the conventional input impedance-matching network 120 comprises inductances LG1 and LG2 and capacitor CMOSCAP. Similarly, the conventional output impedance-matching network 140 comprises inductor LD1, LD2, and LD3 and capacitors CShuntCAP and CBondback which represent the capacitances of discrete shunt capacitors. The capacitance value of CBondback is usually tens of times larger than the capacitance value of CShuntCAP. The leg comprising the series connection of the inductor LD2 and the capacitor CShuntCAP functions like a pure inductor at the fundamental frequency ω, while the leg comprising the series connection of the inductor LD3 and the capacitor CBondback functions like a pure capacitor at the fundamental frequency ω. The output impedance ZL_Die of the RF power amplifier 100 die is given by Equation (1) as follows:
                              Z                      L            Die                          =                              (                                          R                                  D                  ⁢                                                                          ⁢                  2                                            +                              j                ⁡                                  (                                                            ω                      ⁢                                                                                          ⁢                                              L                                                  D                          ⁢                                                                                                          ⁢                          2                                                                                      -                                          1                                              ω                        ⁢                                                                                                  ⁢                                                  C                          ShuntCAP                                                                                                      )                                                      )                    //                                                 [                                                                    (                                                                  R                                                  D                          ⁢                                                                                                          ⁢                          1                                                                    +                                              j                        ⁢                                                                                                  ⁢                        ω                        ⁢                                                                                                  ⁢                                                  L                                                      D                            ⁢                                                                                                                  ⁢                            1                                                                                                                )                                    +                                      (                                                                  R                                                  D                          ⁢                                                                                                          ⁢                          3                                                                    +                                              j                        ⁡                                                  (                                                                                    ω                              ⁢                                                                                                                          ⁢                                                              L                                                                  D                                  ⁢                                                                                                                                          ⁢                                  3                                                                                                                      -                                                          1                                                              ω                                ⁢                                                                                                                                  ⁢                                                                  C                                  Bondback                                                                                                                                              )                                                                                      )                                                  //                                  Z                                      L                    ⁢                                                                                  ⁢                    _                    ⁢                                                                                  ⁢                    PCB                                                              ]                                                          (        1        )            where:                ω is the frequency (in radians per second) of the amplified RF signal 135;        RD1, RD2, and RD3 are the inherent resistances associated with the inherent inductors LD1, LD2, and LD3, respectively; and        ZL_PCB is the circuit impedance seen by the RF power amplifier 100 at its output node 150, which is substantially dependent on the downstream component(s) configured, for example, on a printed circuit board (PCB) on which the RF power amplifier chip is mounted.        
In some implementations, the resonant frequency of the series combination of the inductance LD3 and the capacitance CBondback substantially matches the second harmonic frequency of the amplified RF signal 135. In that case, the RF power amplifier 100 will operate with both reduced power and reduced efficiency.